Embodiments of the present invention pertain to integrated circuits. Specifically, embodiments of the present invention pertain to bandgap and sub-bandgap reference circuits.
Many contemporary CMOS (complementary metal-oxide silicon) integrated circuit chips contain a large digital core along with some peripheral analog circuitry. The analog circuitry typically includes reference voltage circuits that are relied upon by various analog blocks and/or by select digital circuits. These reference voltage circuits should optimally provide a stable, dependable and accurate reference voltage.
One of the most widely adopted reference voltage circuits is referred to as a xe2x80x9cbandgapxe2x80x9d circuit. The bandgap circuit is based on an established physical phenomenon exhibited by silicon. Basically, silicon has a bandgap potential of 1.21 volts. The bandgap potential of silicon can be exploited to produce an extremely reliable and tight reference voltage.
According to the prior art, in order to produce a bandgap reference voltage of 1.21 volts, an operating supply voltage of 1.5 volts or greater is typically required in order to provide a margin of overhead. The majority of analog CMOS circuits today operate at a voltage of three (3) volts, which amply meets the needs of conventional bandgap circuits. However, advances in technology that have resulted in smaller and faster digital circuitry are pushing analog counterparts to keep pace. This, combined with a desire to reduce the voltage and current (i.e., power) requirements, is pushing analog circuitry to operate at voltages as low as one (1) volt, and perhaps even less than 1 volt. Quite obviously, this is less than the bandgap potential of 1.2 volts. To reduce the operating supply voltage to below the bandgap potential, sub-bandgap circuits that can provide a stable reference voltage with an operating supply voltage less than the bandgap potential are also being realized.
Previously, bandgap and sub-bandgap circuits have been realized with combinations of capacitors, diodes, bipolar devices, and resistors. Cost pressures coupled with the process complexity of deep sub-micron technology (e.g., less than 0.12 micron) are starting to make the use of passive devices prohibitive. Moreover, passive devices generally tend to reduce the speed of the circuit and are relatively large. Accordingly, bandgap and sub-bandgap circuits that provide a solution to these problems would be desirable.
Embodiments of the present invention pertain to a reference circuitxe2x80x94specifically, a precision CMOS circuitxe2x80x94that uses field effect transistors (FETs) in lieu of resistors and diodes. A first plurality of FETs is coupled in series, source node to drain node. A second plurality of FETs is also coupled in series, source node to drain node. The source node of a FET in the second plurality of FETs is coupled to the gate node of a respective FET in the first plurality of FETs. The gate node of each FET in the second plurality of FETs is coupled to ground such that a specified total voltage drop across the first plurality of FETs is realizable. The combination of the first and second plurality of FETs are usable as a replacement for a resistor. The circuit can also include a FET configured so that it is usable as a replacement for a diode. Accordingly, parasitics that can affect circuit performance are minimized. Furthermore, with the drive to smaller scale processes and faster circuits, future technologies are making analog design more difficult, and the use of FETs in place of diodes, resistors and capacitors helps alleviate that difficulty.